Lync Server Application Pool on 2013 FE’s Crashes- Why?


by Steve Schiemann, UC Escalation Engineer – Microsoft

Reviewed by – Jason Epperly, Windows Escalation Engineer, Microsoft


I am a Skype for Business Escalation Engineer. This means that I see some of the most interesting problems reported by customers. This is one of those issues that I just HAD to write about, because of the odd twists and turns encountered on the way to producing a fix. This is a high-level view of the issue, in that many technical details were left out. But I hope it leads to a better understanding of how a bug actually gets fixed through our internal processes, and why at times, it may seem like progress is not as fast as we (or customers) would like.



One of Microsoft’s Premier customers created a support case, because several of the application pools on multiple Lync 2013 Front End Servers would crash several times per week, thus impacting service to end users. These affected processes included the DataMCUSvc, IMMCUSvc, ASMCUSvc, and also (less frequently) RtcHost.exe.



The customer saw an event IDs 1000 and 1026 in quick succession in the application log, for example:

Time: 12/15/2015 2:13:57 AM

ID: 1000

Level: Error

Source: Application Error


Message: Faulting application name: DataMCUSvc.exe, version: 5.0.8308.0, time stamp: 0x5050e3de

Faulting module name:, version: 4.0.30319.36331, time stamp: 0x561e0e38

Exception code: 0xc0000005

Fault offset: 0x00000000005450d4

Faulting process id: 0x23d8

Faulting application start time: 0x01d1367cd6e190fb

Faulting application path: D:\Program Files\Microsoft Lync Server 2013\Web Conferencing\DataMCUSvc.exe

Faulting module path: C:\WINDOWS\assembly\NativeImages_v4.0.30319_64\mscorlib\322be87054b632752961a02ac84a27c7\

Report Id: c99ad6ad-a303-11e5-80f9-005056a309a8

Time: 12/15/2015 2:13:57 AM

ID: 1026

Level: Error

Source: .NET Runtime


Message: Application: DataMCUSvc.exe

Framework Version: v4.0.30319

Description: The process was terminated due to an unhandled exception.

Exception Info: System.NullReferenceException


at System.Threading._IOCompletionCallback.PerformIOCompletionCallback(UInt32, UInt32, System.Threading.NativeOverlapped*)

User-mode memory dumps were also sent in, from the crashing processes. We had to use special syntax with procdump, to get the context of the crashes:

procdump -ma -e 1 -f NullReferenceException -accepteula DataMCUSvc.exe

This syntax allowed us to capture the dump at the time of the first-chance NullReferenceException, which correlates to the “Exception Info” from the event ID 1026.


Analysis and First-Chance Exception (mine)

I’m not going to go deeply into the analysis of the memory dumps, but here is the faulting call stack:


0:027> kcn

# Call Site

00 ntdll!ZwWaitForSingleObject

01 KERNELBASE!WaitForSingleObjectEx

02 clr!CLREventWaitHelper2

03 clr!CLREventWaitHelper

04 clr!CLREventBase::WaitEx

05 clr!Thread::WaitSuspendEventsHelper

06 clr!Thread::WaitSuspendEvents

07 clr!Thread::RareEnablePreemptiveGC

08 clr!Thread::EnablePreemptiveGC

09 clr!Thread::RareDisablePreemptiveGC

0a clr!Thread::DisablePreemptiveGC

0b clr!EEDbgInterfaceImpl::DisablePreemptiveGC

0c clr!GCHolderEEInterface<0,1,1>::LeaveInternal

0d clr!GCHolderEEInterface<0,1,1>::{dtor}

0e clr!Debugger::SendExceptionHelperAndBlock

0f clr!Debugger::SendExceptionEventsWorker

10 clr!Debugger::SendException

11 clr!Debugger::FirstChanceManagedException

12 clr!EEToDebuggerExceptionInterfaceWrapper::FirstChanceManagedException

13 clr!ExceptionTracker::ProcessManagedCallFrame

14 clr!ExceptionTracker::ProcessOSExceptionNotification

15 clr!ProcessCLRException

16 ntdll!RtlpExecuteHandlerForException

17 ntdll!RtlDispatchException

18 ntdll!KiUserExceptionDispatch

19 KERNELBASE!RaiseException

1a clr!NakedThrowHelper2

1b clr!NakedThrowHelper_RspAligned

1c clr!zzz_AsmCodeRange_End

1d mscorlib_ni!System.Threading._IOCompletionCallback.PerformIOCompletionCallback(UInt32, UInt32, System.Threading.NativeOverlapped*)

1e clr!CallDescrWorkerInternal

1f clr!CallDescrWorkerWithHandler

20 clr!DispatchCallSimple

21 clr!BindIoCompletionCallBack_Worker

22 clr!ManagedThreadBase_DispatchInner

23 clr!ManagedThreadBase_DispatchMiddle

24 clr!ManagedThreadBase_DispatchOuter

25 clr!ManagedThreadBase_FullTransitionWithAD

26 clr!ManagedThreadBase::ThreadPool

27 clr!BindIoCompletionCallbackStubEx

28 clr!BindIoCompletionCallbackStub

29 clr!ThreadpoolMgr::CompletionPortThreadStart

2a clr!Thread::intermediateThreadProc

2b kernel32!BaseThreadInitThunk

2c ntdll!RtlUserThreadStart

Based on dump analysis, the root cause was far from clear, but it seemed that the problem was with the Common Language Runtime (CLR). There isn’t even a Lync Server module present in this stack! The issue continued even after the customer updated to the latest .NET patches. I mean, the event IDs above pointed to a .NET problem. I could also see an error code associated with the first chance exception: E0434F4DSystem.NullReferenceException

Looking this error code up with err.exe:

0x80131506 (2148734214): COR_E_EXECUTIONENGINE – corerror.h: An internal error happened in the Common Language Runtime&apos;s Execution Engine

OK, this is a .NET bug! Microsoft Customer Service and Support (CSS) has specialists for just about every product we produce, so I engaged the .NET team for input. Now I can just let them do the heavy lifting and fix their component! Wrong.

The .Net team found no issue with their product. As a matter of fact, they said that this was clearly a Lync Server issue. Lync Server code has not properly synchronized the access to the OVERLAP structure, perhaps double-freeing the NativeOverlapped object. Nothing to see here, move along. Rats!


Next Step

After extensive debugging by myself and the .NET team, there was still no clear cause for the crashes, and no resolution for the customer. So, I engaged the Skype for Business (SfB, formerly Lync) product group (PG) by opening a bug. I stated the symptoms, analysis done so far by all parties, and the .NET team’s suspicions that this was not their problem, but ours.

Fortunately, one of our excellent SfB developers had spent some time developing/debugging core Windows applications. He was able to identify the issue, even without a kernel dump. He had seen it before with a different Lync Server issue. In that case, the PG was able to code around the problem even though it was not truly a Skype for Business problem. With these app pool crashes, the fix had to come from the source component, which was NOT part of Lync Server code.


Root Cause at Last!

This was a Windows problem, that bubbled up through .NET and hit Lync Servers. Short story, the Windows I/O Manager, starting with Vista, has the ability to use thread agnostic I/O. This means that the original thread which issued the I/O no longer has to be present when the I/O it issues completes. In our case, the FO_QUEUE_IRP_TO_THREAD flag on the FILE_OBJECT structure effectively disables this feature, and queues the I/O Request Packet (IRP) to the issuing thread. The status code returned to the issuing thread by CloseHandle was incorrect; the code returned was STATUS_BUFFER_OVERFLOW, rather than STATUS_PENDING. This caused processes on Lync Servers to see unexpected double completions, which resulted in the crashes.


The Road to Releasing a Fix

So, now that the root cause was understood, it was time to engage our Windows support team, and ask them to open a bug with THEIR product group, and get this fixed. I did this, and based on the strong Business Impact Statement (BIS) from my customer, and the fact that several more cases had been opened for this issue, a bug was filed, and the Windows PG agreed to fix this. Yay! Not so fast…first, the problem was also in the next Windows Server version (2016), that wasn’t even released yet. The fix had to be “baked” or tested in our internal labs on Server 2016 for several weeks, and if all went well, backported to Server 2012 R2. The Lync 2013 Server crash cases that had come in were all running this version.

Unfortunately, the first version of the fix did not fully resolve the issue, so the stress tests on Server 2016 had to be restarted, and run again. This time, there was no issue with the fix, and it was backported to Server 2012 R2.


Finding the Fix

It is within this rollup:

3179574 August 2016 update rollup for Windows RT 8.1, Windows 8.1, and Windows Server 2012 R2

You can verify that it is installed by looking for KB3179574 in add/remove programs. There is no individual KB for the issue at this time, but it is listed as one of the fixes in the rollup see below.

CrashBlog1 CrashBlog2


At Last

The whole process, from the opening the first case, to the release of a public fix, took almost a year. During this time, several support teams and product groups were involved, customers went on vacation, private versions of the fix were tested by various customers, and internal tests failed. But finally, a solid fix is publicly available and this problem should not be seen anymore. We strive daily to make our products better, and need to improve our processes so that customers see resolution to similar issues more quickly. I hope this article gives some insight as to what goes on when a code change is needed in a product- sometimes the product that exhibits the symptom is not really at fault.

Why did this problem affect Lync 2013 Front-End (FE) servers exclusively? Well, these FE servers use the .NET HTTP Listener API to asynchronously retrieve a client certificate. Our certificates tend to be rather large, and do not fit into the default buffer. This results in the asynchronous completion returning STATUS_BUFFER_OVERFLOW.  When STATUS_BUFFER_OVERFLOW is returned to the caller, it expects no asynchronous completion. The fix was to return STATUS_PENDING instead.

Comments (1)

  1. Dru Walstrom says:

    Seeing the same kind of error, only with the Centralized Logging Agent. Applying that update doesn’t fix it though.

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